Talk:Magnetization

Permeability

Putting Corsan and Loraine in relative units, I would write that

$\mathbf {B} =\mu _{0}\mu (\mathbf {H+M} )$

or do you mean that $\mu _{0}$ is the permeability? David R. Ingham 01:45, 26 August 2006 (UTC)

The equation above is not correct: $\mu$ should not be in the equation. In SI units this equation should read

$\mathbf {B} =\mu _{0}\mu _{r}\mathbf {H} =\mu _{0}(\mathbf {H+M} )$.

Here $\mu _{0}$ is the permeability of the vacuum ($\mu _{0}=4\pi 10^{-7}$ H/m) and $\mu _{r}$ is the relative permeability of the material. Assuming that

$\mathbf {M} =\chi _{r}\mathbf {H}$

which is correct for most materials in the world (not valid for ferromagnets) we obtain that

$\mathbf {B} =\mu _{0}\mu _{r}(\mathbf {H+\chi _{r}\mathbf {H} } )=\mu _{0}(1+\chi _{r})\mathbf {H}$

thus $\mu _{r}=1+\chi _{r}$. In this equation $\chi _{r}$ is called the relative susceptibility.

TomyDuby 12:13, 29 December 2006 (UTC)

I think this should be:

$\mathbf {B} =\mu _{0}(\mathbf {H+\chi _{r}\mathbf {H} } )=\mu _{0}(1+\chi _{r})\mathbf {H}$

84.67.65.165 (talk) 16:24, 25 April 2008 (UTC)

nanomagnetism

The comment on nanomagnetism contains neither information on what nanomagnetism is nor the significance of the findings. It seems to be crude commercial from University of the Basque Country. Could somebody either add information or delete it? --Frozenport (talk) 14:40, 30 June 2008 (UTC)

Yea, I deleted it. I'm sure it's fine science, but not so important that it's on par with paramagnetism, etc. Scientific press releases are notorious for exaggerating importance, and (in a brief search) I couldn't find any third-party coverage of the topic, let alone large amounts of it. --Steve (talk) 15:27, 30 June 2008 (UTC)

Magnetization of Radio waves

Magnetization of Radio waves is the process by which some magnets while expose to radio waves can convert those waves into electrical energy. The process can best be understood by imagining to magnets set side by side and touching opposite poles, but fer enough so that radio waves can enter and bounce back and fort through their magnetic fields. The polarized fields of the poles will keep the waves bouncing back and forth and eventually these will accelerate until finally becoming statically charge. This process might be most commonly know as electromagnetism and was first observed by the scientist James Clerk Maxwell.—Preceding unsigned comment added by 12.87.73.114 (talk) 05:44, 6 March 2010 (UTC)

Specific magnetic moment

I think the specific magnetic moment should be added.

I am having trouble sorting out the connections between M in the formula $\mathbf {B} =\mu _{0}(\mathbf {H+M} )$ given and the Magnetization which is reported in Am^2/kg. The magnetic field H is measured in A/m. Then, of course M should also be in A/m so that when multiplying the sum with mu0 (in T*m/A) the magnetic flux B is in Tesla. However, I do not find how to get from the specific magnetization to M in A/m.

The specific magnetization is given in Gauss cm^3/g which is equal to 1*10^-7 Wb*m/kg.

So how does one use this specific magnetic moment of a material to compute the magnetization M and thus B? — Preceding unsigned comment added by 134.94.244.46 (talk) 11:27, 6 December 2011 (UTC)

Some people report the magnetization in Am^2/kg, but many report it in A/m, consistent with the formula. Just multiply the former by the density of the material to get the latter. RockMagnetist (talk) 17:14, 6 December 2011 (UTC)

Indeed. The Am^2/kg unit is the equivalent of older (but still widely used) emu/g. However, magnetisation is the total magnetic moment related to volume, NOT mass. Different substances have different mass per unitary volume, so conversion of Am^2/kg (per mass) to A/m (per volume) must include specific mass, i.e. density. --JPFen (talk) 14:08, 6 August 2013 (UTC)

Demagnetization vs. Degaussing

Are you sure that effect described here in sections "Demagnetization" and "Applications of Demagnetization" is not in fact Degaussing? Demagnetization is often used for lowering of magnetic field inside open samples due to the presence of Demagnetizing field.

These two effects should not be confused. 147.231.26.31 (talk) 15:25, 19 May 2014 (UTC)

Actually, the way these terms are generally used, degaussing is a type of demagnetization that involves applying a steady field. There are other demagnetizing methods, for example applying an alternating magnetic field or heating the material, that are not usually called degaussing. Demagnetizing field is something else entirely - the field due to the magnetization in a material. It does tend to reduce the magnetization, but an external field must be applied to completely demagnetize it. RockMagnetist (talk) 15:51, 19 May 2014 (UTC)

But look at Degaussing#Monitors (and the next section about storage media as well) - applying an AC magnetic field with decreasing amplitude is called degaussing there, contrary to your explanation. 147.231.26.31 (talk) 13:54, 20 May 2014 (UTC)

I stand corrected. In my field, we use "demagnetization" for all of the above methods, and I can easily provide several sources to support that claim. We rarely use the term "degaussing", so my opinion was based on a quick web search. Maybe it is also used for thermal demagnetization. However, at most degaussing is a synonym for demagnetization. RockMagnetist (talk) 14:46, 20 May 2014 (UTC)

Relation of Magnetization to the Free Energy

I believe it would be worthy to add a definition of the magnetization with respect to the free energy. This would highlight the fact that magnetization is on the same thermodynamic level as other first derivatives of the free energy (like entropy, pressure, and chemical potential). This is thus one reason why it can be useful to measure the magnetization of a material.

The magnetization with respect to the free energy is given on the Helmholtz free energy page as $m=-{\bigg (}{\frac {\partial F}{\partial B}}{\bigg )}{\bigg |}_{T,N}.$ This can be obtained by adding $-MdB$ to the free energy (as is discussed in Ref.). ThePaulanator (talk) 03:39, 24 October 2019 (UTC)

References

Sure be free to add it.--ReyHahn (talk) 10:44, 24 October 2019 (UTC)

Okay, just added it! I may have messed up on volume magnetization vs. total magnetization. ThePaulanator (talk) 22:26, 25 October 2019 (UTC)

Reverted edits

@ThePaulanator: hi, I just reverted your edit but I'll be welcome to add it back if we solve the following inconsistency: it was written that

$dF=-SdT-{\textbf {M}}d{\textbf {B}}$

this equation cannot be right, just check the units. The free energy F has units of energy, SdT too, but MdB is missing a volume factor. That's why I removed it, it is inconsistent and can lead to generalized errors. The only possibility, is that it is either

$dF=-SdT-{\frac {1}{V}}{\textbf {M}}d{\textbf {B}}$

or

$dF=-SdT-{\textbf {m}}d{\textbf {B}}$

where m is the magnetic moment. I will go for the second one, as the energy given by magnetic moments is indeed $-\mathbf {m} \cdot \mathbf {B}$ without M. Also the dependence between M and m might not be trivial in general. So (in this case) the actual formula is

${\textbf {M}}=-{\frac {\partial }{\partial V}}\left(\left({dF \over d{\textbf {B}}}\right)_{T,V,N}\right)$.

I will try to check the reference, but some authors use M for the magnetic moment (or call m the magnetization).--ReyHahn (talk) 09:13, 14 January 2021 (UTC)

Both sources you cited, [1] and Blundell, are doing the same mistake. The first pdf is calling it magnetic moment, but then uses the same letter for magnetization. Blundell says that the energy is μdB but then writes M. Michael Coey Magnetism and Magnetic Materials has a more clear discussion and clearly uses magnetic moment when discussing the free energy.

I think the best would be to move that text to magnetic moment and change M for m.ReyHahn (talk) 14:47, 14 January 2021 (UTC)

Hi @ReyHahn, you are correct there is a volume term missing (I think these sources often implicitly volume normalize all the extrinsic quantities to be intrinsic quantities). I will add to the magnetic moment page. 18.10.93.56 (talk) 15:31, 19 May 2022 (UTC)

Physics

Define Magnetisation 27.34.47.188 (talk) 14:25, 19 May 2022 (UTC)